Linkedin is currently the most widely used and largest professional and business network in the world. It allows you to search for jobs, introduce yourself, post job offers and business opportunities.
Note that it’s possible to log in and sign up with Google.
Viadeo is a European social network mainly with France as the main market. Similar to linkedin, it enables a lot of business-based possibilities.
Viadeo has approached the “Journal du Net” and has evolved a lot, you can also consult this page: Other Viadeo Link